1. Field of the Invention
The field of the invention is mass spectrometry (MS), and more specifically a process and apparatus for using polarized laser light to provide improved control of ion yields during desorption of a sample.
2. Description of the Related Art
Laser desorption ionization mass spectrometry (LDI-MS) of organic molecules and biomolecules provides chemical analysis with great selectivity and sensitivity. Presently available methods generally rely on the interaction of laser radiation with a matrix material or with nanoporous substrates for the production of ions. Examples of these techniques include matrix-assisted laser desorption ionization (MALDI), desorption ionization on silicon (DIOS), and nanostructure-initiator mass spectrometry (NIMS). From laser shot to laser shot, these methods exhibit spontaneous fluctuations in ion yield that can only be controlled by adjusting the fluence delivered to the surface.
Highly confined electromagnetic fields play an important role in the interaction of laser radiation with nanostructures. Near-field optics show great potential in manipulating light on a sub-micron or even on the molecular scale. Nanophotonics takes advantage of structures that exhibit features commensurate with the wavelength of the radiation. Among others it has been utilized for nanoparticle detection, for the patterning of biomolecules and for creating materials with unique optical properties. The latter include laser-induced silicon microcolumn arrays (LISMAs), produced by ultrafast laser processing of silicon surfaces, and are known to have uniformly high absorptance in the 0.2-2.4 μm wavelength range as well as superhydrophobic properties. At sufficiently high laser intensities, the molecules adsorbed on these nanostructures undergo desorption, ionization and eventually exhibit unimolecular decomposition. The resulting ion fragmentation patterns can be used for structure elucidation in mass spectrometry. Manipulation of ion production from biomolecules with photonic structures (i.e., photonic ion sources) based on the laser light-nanostructure interaction, however, has not previously been demonstrated.
Photonic ion sources based on array-type nanostructures, such as laser induced silicon microcolumn arrays (LISMA), can serve as platforms for LDI-MS for detection of various organic and biomolecules. Compared to conventional LDI-MS ion sources, e.g., MALDI, DIOS and NIMS, nanophotonic ion sources couple the laser energy to the nanostructures via a fundamentally different mechanism due to the quasiperiodic or periodic and oriented nature of the arrays. The inventors have demonstrated for the first time that nanophotonic ion sources show a dramatic disparity in the efficiency of ion production depending on the polarization angle and the angle of incidence of the laser. When the electric field of the radiation has a component that is parallel to the column axes (p-polarized beam) the desorption and ionization processes are efficient, whereas in case they are perpendicular (s-polarized waves) minimal or no ion production is observed. In addition, LISMA structures also exhibit a strong directionality in ion production. The ion yield as a function of the incidence angle of an unpolarized laser beam decreases and ultimately vanishes as the incidence angle approaches 0°. This strong directionality in ion production is a unique feature of these nanostructures.
Photonic ion sources, such as LISMA, rely on the quasiperiodic or periodic and oriented nature of the nanostructures with dimensions commensurate with the wavelength of the laser light. These photonic ion sources rely on unique nanophotonic interactions (e.g., near-field, confinement, and interference effects) between the electromagnetic radiation and the nanostructure on one hand, and the interaction of both with the surface-deposited sample molecules, on the other. These devices exhibit a control of ion production by varying laser radiation properties other than simple pulse energy, mainly through changes in the angle of incidence and the plane of polarization of the laser radiation. Structural parameters of the photonic ion sources (i.e., column diameter, height and periodicity) enable further control of coupling the laser energy on a micro and nano scale. Combination of nanophotonic ion sources with miniaturized mass analyzers can lead to the development of integrated miniaturized mass spectrometers and analytical sensors.